arXiv subjects

Eulogio Oset

Publications and source records attributed to Eulogio Oset.

At least 109 records · Page 6Linked to original sources

arXiv2018

Theoretical analysis on the $K^{-} {}^{3} \text{He} \to Λp n$ reaction for the $\bar{K} N N$ bound-state search in the J-PARC E15 experiment

We theoretically analyze the $K^{-} {}^{3} \text{He} \to Λp n$ reaction for the $\bar{K} N N$ bound-state search in the J-PARC E15 experiment. We find that, by detecting a fast and forward neutron in the final state, an almost on-shell $\bar{K}$ is guaranteed, which is essential to make a bound state with two nucleons from ${}^{3} \text{He}$. Then, this almost on-shell $\bar{K}$ can bring a signal of the $\bar{K} N N$ bound state in the $Λp$ invariant-mass spectrum, although it inevitably brings a kinematic peak above the $\bar{K} N N$ threshold as well. As a consequence, we predict two peaks across the $\bar{K} N N$ threshold in the spectrum: the lower peak coming from the $\bar{K} N N$ bound state, and the higher one originating from the kinematics.

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arXiv2018

Hidden charm pentaquark and $Λ(1405)$ in the $Λ^0_b \to η_c K^- p (πΣ)$ reaction

We have performed a study of the $Λ^0_b \to η_c K^- p$ and $Λ^0_b \to η_c πΣ$ reactions based on the dominant Cabibbo favored weak decay mechanism. We show that the $K^- p$ produced only couples to $Λ^*$ states, not $Σ^*$ and that the $πΣ$ state is only generated from final state interaction of $\bar{K}N$ and $ηΛ$ channels which are produced in a primary stage. This guarantees that the $πΣ$ state is generated in isospin $I=0$ and we see that the invariant mass produces a clean signal for the $Λ(1405)$ of higher mass at $1420$ MeV. We also study the $η_c p$ final state interaction, which is driven by the excitation of a hidden charm resonance predicted before. We relate the strength of the different invariant mass distributions and find similar strengths that should be clearly visible in an ongoing LHCb experiment. In particular we predict that a clean peak should be seen for a hidden charm resonance that couples to the $η_c p$ channel in the invariant $η_c p$ mass distribution.

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arXiv2017

On the chiral covariant approach to $ρρ$ scattering

We examine in detail a recent work (D.~Gülmez, U.-G.~Meißner and J.~A.~Oller, Eur. Phys. J. C 77:460 (2017)), where improvements to make $ρρ$ scattering relativistically covariant are made. The paper has the remarkable conclusion that the $J=2$ state disappears with a potential which is much more attractive than for $J=0$, where a bound state is found. We trace this abnormal conclusion to the fact that an "on-shell" factorization of the potential is done in a region where this potential is singular and develops a large discontinuous and unphysical imaginary part. A method is developed, evaluating the loops with full $ρ$ propagators, and we show that they do not develop singularities and do not have an imaginary part below threshold. With this result for the loops we define an effective potential, which when used with the Bethe-Salpeter equation provides a state with $J=2$ around the energy of the $f_2(1270)$. In addition, the coupling of the state to $ρρ$ is evaluated and we find that this coupling and the $T$ matrix around the energy of the bound state are remarkably similar to those obtained with a drastic approximation used previously, in which the $q^2$ terms of the propagators of the exchanged $ρ$ mesons are dropped, once the cut-off in the $ρρ$ loop function is tuned to reproduce the bound state at the same energy.

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arXiv2017

Analysis of the $B^+\to J/ψϕK^+$ data at low $J/ψϕ$ invariant masses and the $X(4140)$ and $X(4160)$ resonances

We have studied the $J/ψϕ$ mass distribution of the $B^+\to J/ψϕK^+$ reaction from threshold to about 4250 MeV, and find that one needs the contribution of the $X(4140)$ with a narrow width, together with the $X(4160)$ which accounts for most of the strength of the distribution in that region. The existence of a clear cusp at the $D_s^* \bar{D}_s^*$ threshold indicates that the $X(4160)$ resonance is strongly tied to the $D_s^* \bar{D}_s^*$ channel, which finds a natural interpretation in the molecular picture of this resonance.

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arXiv2017

What makes the peak structure of the $Λp$ invariant-mass spectrum in the $K^{-} {}^{3} {\rm He} \to Λp n$ reaction?

Recently a peak structure was observed near the $K^{-} p p$ threshold in the in-flight ${}^{3} {\rm He} (K^{-} , \, Λp) n$ reaction of the E15 experiment at J-PARC, which could be a signal of a $\bar{K} N N$ bound state. In order to investigate what is the origin of this peak, we calculate the cross section of this reaction, in particular based on the scenario that the $\bar{K} N N$ bound state is indeed generated and decays into $Λp$. We find that the numerical result of the $Λp$ invariant-mass spectrum in the $\bar{K} N N$ bound scenario is consistent with the J-PARC E15 data.

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arXiv2017

Triangle mechanisms in the build up and decay of the $N^*(1875)$

We have studied the $N^*(1875) (3/2^-)$ resonance with a multichannel unitary scheme, considering the $Δπ$ and $Σ^* K$, with their interaction extracted from chiral Lagrangians, and then have added two more channels, the $N^*(1535) π$ and $N σ$, which proceed via triangle diagrams involving the $Σ^* K$ and $Δπ$ respectively in the intermediate states. The triangle diagram in the $N^*(1535) π$ case develops a singularity at the same energy as the resonance mass. We determine the couplings of the resonance to the different channels and the partial decay widths. We find a very large decay width to $Σ^* K$, and also see that, due to interference with other terms, the $N σ$ channel has an important role in the $ππ$ mass distributions at low invariant masses, leading to an apparently large $N σ$ decay width. A discussion is done, justifying the convenience of an experimental reanalysis of this resonance to the light of the findings of the paper, using multichannel unitary schemes.

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arXiv2017

Theoretical study of the Xi(1620) and Xi(1690) resonances in Xi_c -> pi^+ MB decays

Nonleptonic weak decays of Xi_c into pi^+ and a meson (M)-baryon (B) final state, MB, are analyzed from the viewpoint of probing S=-2 baryon resonances, i.e. Xi(1620) and Xi(1690), of which spin-parity and other properties are not well known. We argue that the weak decay of Xi_c is dominated by a single quark-line diagram, preferred by the Cabibbo-Kobayashi-Maskawa coefficient, color recombination factor, the diquark correlation, and the kinematical condition. The decay process has an advantage of being free from meson resonances in the pi^+M invariant mass distribution. The invariant mass distribution of the meson-baryon final state is calculated with three different chiral unitary approaches, assuming that the Xi(1620) and Xi(1690) resonances have J^P=1/2^-. It is found that a clear peak for the Xi(1690) is seen in the piXi and KbarLambda spectra. We also suggest that the ratios of the piXi, KbarLambda and KbarSigma final states are useful to distinguish whether the peak is originated from the Xi(1690) resonance or it is a KbarSigma threshold effect.

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arXiv2017

$f_2(1810)$ as a triangle singularity

We perform calculations showing that a source producing $K^* \bar{K}^*$ in $J = 2$ and $L=0$ gives rise to a triangle singularity at $1810$ MeV with a width of about $200$ MeV from the mechanism $K^* \to πK$ and then $K\bar{K}^*$ merging into the $a_1(1260)$ resonance. We suggest that this is the origin of the present $f_2(1810)$ resonance and propose to look at the $πa_1(1260)$ mode in several reactions to clarify the issue.

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arXiv2017

Role of a triangle singularity in the $γp\rightarrow K^+ Λ(1405)$ reaction

We show the effects of a triangle singularity mechanism for the $γp \to K^+Λ(1405)$ reaction. The mechanism has a $N^*$ resonance around 2030 MeV, which decays into $K^* Σ$. The $K^*$ decays to $K^+ π$, and the $πΣ$ merge to form the $Λ(1405)$. This mechanism produces a peak around $\sqrt{s} = 2110$ MeV, and has its largest contribution around cos$θ=0$. The addition of this mechanism to other conventional ones, leads to a good reproduction of ${\rm d}σ/{\rm dcos}θ$ and the integrated cross section around this energy, providing a solution to a problem encountered in previous theoretical models.

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arXiv2016

Semileptonic $Λ_b \to \bar ν_l l Λ_c(2595)$ and $Λ_b \to \bar ν_l l Λ_c(2625)$ decays in the molecular picture of $Λ_c(2595)$ and $Λ_c(2625)$

We evaluate the partial decay widths for the semileptonic $Λ_b \to \bar ν_l l Λ_c(2595)$ and $Λ_b \to \bar ν_l l Λ_c(2625)$ decays from the perspective that these two $Λ^*_c$ resonances are dynamically generated from the $DN$ and $D^*N$ interaction with coupled channels. We find that the ratio of the rates obtained for these two reactions is compatible with present experimental data and is very sensitive to the $D^* N$ coupling, which becomes essential to obtain agreement with experiment. Together with the results obtained for the $Λ_b \to π^- Λ^*_c$ reactions, it gives strong support to the molecular picture of the two $Λ^*_c$ resonances and the important role of the $D^*N$ component neglected in prior studies of the $Λ_c(2595)$ from the molecular perspective.

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arXiv2016

The Peak Structure in the In-Flight ${}^{3}\text{He} ( K^{-} , \, Λp ) n$ Reaction Around the $\bar{K} N N$ Threshold

We theoretically investigate the origin of the peak structure around the $K^{-} p p$ threshold observed in the in-flight ${}^{3}\text{He} ( K^{-} , \, Λp ) n$ reaction in the recent E15 experiment at J-PARC. For this purpose, we consider two scenarios to produce the peak. One is that the $Λ(1405)$ is generated but it does not correlate with $p$, and the uncorrelated $Λ(1405) p$ system subsequently decays into $Λp$. The other one is that the $\bar{K} N N$ bound state is indeed generated and decays into $Λp$. As a result, the experimental signal is qualitatively well reproduced in the $\bar{K} N N$ bound scenario, definitely discarding the uncorrelated $Λ(1405) p$ one.

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arXiv2016

$Λ_b \to π^- (D_s^-) Λ_c(2595),~π^- (D_s^-) Λ_c(2625)$ decays and $DN,~D^*N$ molecular components

From the perspective that the $Λ_c(2595)$ and $Λ_c(2625)$ are dynamically generated resonances from the $DN,~D^*N$ interaction and coupled channels, we have evaluated the rates for $Λ_b \to π^- Λ_c(2595)$ and $Λ_b \to π^- Λ_c(2625)$ up to a global unknown factor that allows us to calculate the ratio of rates and compare with experiment, where good agreement is found. Similarly, we can also make predictions for the ratio of rates of the, yet unknown, decays of $Λ_b \to D_s^- Λ_c(2595)$ and $Λ_b \to D_s^- Λ_c(2625)$ and make estimates for their individual branching fractions.

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arXiv2016

On the structure observed in the in-flight ${}^{3}\text{He} ( K^{-} , \, Λp ) n$ reaction at J-PARC

A theoretical investigation is done to clarify the origin of the peak structure observed near the $K^{-} p p$ threshold in the in-flight ${}^{3}\text{He} ( K^{-}, \, Λp ) n$ reaction of the J-PARC E15 experiment, which could be a signal of the lightest kaonic nuclei, that is, the $\bar{K} N N (I=1/2)$ state. For the investigation, we evaluate the $Λp$ invariant mass spectrum assuming two possible scenarios to interpret the experimental peak. One assumes that the $Λ(1405)$ resonance is generated after the emission of an energetic neutron from the absorption of the initial $K^-$, not forming a bound state with the remaining proton. This uncorrelated $Λ(1405) p$ system subsequently decays into the final $Λp$. The other scenario implies that, after the emission of the energetic neutron, a $\bar{K} N N$ bound state is formed, decaying eventually into a $Λp$ pair. Our results show that the experimental signal observed in the in-flight ${}^{3}\text{He} ( K^{-} , \, Λp ) n$ reaction at J-PARC is qualitatively well reproduced by the assumption that a $\bar{K} N N$ bound state is generated in the reaction, definitely discarding the interpretation in terms of an uncorrelated $Λ(1405) p$ state.

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arXiv2016

Determination of the $\boldsymbol{η^3{\rm He}}$ threshold structure from the low energy $\boldsymbol{pd \to η^3{\rm He}}$ reaction

We analyze the data on cross sections and asymmetries for the $pd (dp) \to η^3{\rm He}$ reaction close to threshold and look for bound states of the $η^3 {\rm He}$ system. Rather than parameterizing the scattering matrix, as is usually done, we develop a framework in which the $η^3 {\rm He}$ optical potential is the key ingredient, and its strength, together with some production parameters, are fitted to the available experimental data. The relationship of the scattering matrix to the optical potential is established using the Bethe-Salpeter equation and the $η^3 {\rm He}$ loop function incorporates the range of the interaction given by the empirical $^3 {\rm He}$ density. We find a local Breit Wigner form of the $η^3$He amplitude $T$ below threshold with a clear peak in $|T|^2$, which corresponds to an $η^3 {\rm He}$ binding of about 0.3 MeV and a width of about 3 MeV. By fitting the potential we can also evaluate the $η^3 {\rm He}$ scattering length, including its sign, thus resolving the ambiguity in the former analyses.

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arXiv2016

A Discussion on Triangle Singularities in the $Λ_b \to J/ψK^{-} p$ Reaction

We have analyzed the singularities of a triangle loop integral in detail and derived a formula for an easy evaluation of the triangle singularity on the physical boundary. It is applied to the $Λ_b \rightarrow J/ψK^{-} p$ process via $Λ^*$-charmonium-proton intermediate states. Although the evaluation of absolute rates is not possible, we identify the $χ_{c1}$ and the $ψ(2S)$ as the relatively most relevant states among all possible charmonia up to the $ψ(2S)$. The $Λ(1890)\, χ_{c1}\, p$ loop is very special as its normal threshold and triangle singularities merge at about 4.45 GeV, generating a narrow and prominent peak in the amplitude in the case that the $χ_{c1}\, p$ is in an $S$-wave. We also see that loops with the same charmonium and other $Λ^*$ hyperons produce less dramatic peaks from the threshold singularity alone. For the case of $χ_{c1}\, p \rightarrow J/ψ\, p$ and quantum numbers $3/2^-$ or $5/2^+$ one needs $P$- and $D$-waves, respectively, in the $χ_{c1}\, p$, which drastically reduce the strength of the contribution and smooth the threshold peak. In this case we conclude that the singularities cannot account for the observed narrow peak. In the case of $1/2^+$, $3/2^+$ quantum numbers, where $χ_{c1}\, p \rightarrow J/ψ\, p$ can proceed in an $S$-wave, the $Λ(1890)\,χ_{c1}\,p$ triangle diagram could play an important role, though can neither assert their strength without further input from experiments and lattice QCD calculations.

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arXiv2016

Novel nonperturbative approach for radiative $\bar{B}^0(\bar{B}^0_s)\rightarrow J/ψγ$ decays

Radiative $\bar{B}^0(\bar{B}^0_s)\rightarrow J/ψγ$ decays provide an interesting case to test our understanding of (non)perturbative QCD and eventually to probe physics beyond the standard model. Recently, the LHCb Collaboration has reported an upper bound, updating the results of the BABAR Collaboration. Previous theoretical predictions based on QCD factorization or perturbative QCD have shown large variations due to different treatment of nonfactorizable contributions and meson-photon transitions. In this paper, we report on a novel approach to estimate the decay rates, which is based on a recently proposed model for $B$ decays and the vector meson dominance hypothesis, widely tested in the relevant energy regions. The predicted branching ratios are $\mathrm{Br}[\bar{B}^0\rightarrow J/ψγ]=\left(3.50\pm0.34^{+1.12}_{-0.63}\right)\times10^{-8}$ and $\mathrm{Br}[\bar{B}^0_s\rightarrow J/ψγ]=\left(7.20\pm0.68^{+2.31}_{-1.30}\right)\times10^{-7}$. The first uncertainty is systematic and the second is statistical, originating from the experimental $\bar{B}^0_s\rightarrow J/ψϕ$ branching ratio.

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arXiv2016

Summary of the 2015 LHCb workshop on multi-body decays of D and B mesons

This document contains a summary of the LHCb workshop on multi-body decays of D and B mesons, held at CBPF, Rio de Janeiro, in July 2015. The workshop was focused on issues related to amplitude analysis of three- and four-body hadronic decays. In addition to selected LHCb results, contributions from guest theorists are included.

hep-ex↗

arXiv2016

A hidden-charm pentaquark state in $Λ^0_b \to J/ψp π^-$ decay

We study here the $Λ_b^0 \to J/ψp π^-$ reaction in analogy to the $Λ^0_b \to J/ψp K^-$ one, and we note that in both decays there is a sharp structure (dip or peak) in the $J/ψp$ mass distribution around $4450$ MeV, which is associated in the $Λ^0_b \to J/ψp K^-$ experiment to an exotic pentaquark baryonic state, although in $Λ_b^0 \to J/ψp π^-$ it shows up with relatively low statistics. We analyze the $Λ^0_b \to J/ψp π^-$ interaction along the same lines as the $Λ^0_b \to J/ψp K^-$ one, with the main difference stemming from the reduced Cabibbo strength in the former and the consideration of the $π^-p$ final state interaction instead of the $K^-p$ one. We find that with a minimal input, introducing the $π^-p$ and $J/ψp$ interaction in $S$-wave with realistic interactions, and the empirical $P$-wave and $D$-wave contributions, one can accomplish a qualitative description of the $π^-p$ and $J/ψp$ mass distributions. More importantly, the peak structure followed by a dip of the experimental $J/ψp$ mass distribution is reproduced with the same input as used to describe the data of $Λ^0_b\rightarrow J/ψp K^-$ reaction. The repercussion for the triangular singularity mechanism, invoked in some works to explain the pentaquark peak, is discussed.

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